Variables in Bash

Bash variables hold strings. Assignment has no spaces around =. Use $name or ${name} to read values—compare with Python's clearer assignment rules.

Assignment and echo

name="Ada"
count=3
echo "Hello, $name"
echo "count is ${count}"

Quotes prevent word-splitting on spaces. Without quotes, multiple words become multiple arguments.

Read-only and export

readonly PI=3.14
export APP_ENV=dev
echo "$APP_ENV"

export puts variables into the environment child processes inherit—covered more in env-vars lesson.

Command substitution

today=$(date +%F)
echo "Today is $today"
lines=$(wc -l < notes.txt)
echo "notes.txt has $lines lines"

$(...) runs a command and captures stdout—like running a function that returns text.

Important interview questions and answers

1. Q: Spaces around = in assignment?
   A: Forbidden for simple assignment—name=Ada tries to run command Ada.
2. Q: $name vs ${name}?
   A: Braces disambiguate names: ${file}_backup vs $file_backup.

Self-check

1. Why is name = Ada invalid?
2. What syntax runs date and stores its output?

Pitfall: No spaces around = in assignments—count = 3 runs command =.